We are interested in understanding the regulatory mechanisms by which stimulation of the antigen receptor on T lymphocytes leads to cell proliferation and differentiation. In particular we have been investigating the role of the Src-family tyrosine kinases in the initiation of signal transduction events that follow stimulation of the T cell antigen receptor (TCR). The TCR lacks intrinsic catalytic function and instead relies upon the non-receptor tyrosine kinases for its activity. Previous work has established the importance of Src kinases in the initiation of signaling events following stimulation of the TCR. However, it remains unclear how these kinases carry out their essential role in T cell receptor signal transduction. We will use a variety of approaches to investigate 1) how the function of these kinases are regulated to initiate receptor phosphorylation 2) how they participate in the activation of downstream signaling pathways, and 3) how individual members of the Src family provide specific functions in this signaling pathway. We will take advantage of our ability to express mutants of the Src-family kinase Lck in an Lck-deficient T cell line to determine what steps in the activation process are dependent upon particular functions of Lck. We will extend these studies by determining whether mice that express specific Lck mutants exhibit defects in T cell development or in the activation of mature T cells. In addition we will analyze chimeras constructed between members of the Src kinase family in order to identify those regions that provide specific functions during T cell receptor signal transduction. These experiments will provide insight into how non-receptor tyrosine kinases of the Src family function in the initiation of T cell receptor signal transduction as well as their specific role in the regulation of downstream signaling pathways leading to T cell activation.